Skolithos is a common type of trace fossil that has been found in rocks as old as 541 million years. Trace fossils are not the fossilized remains of organisms but rather the burrows, footprints, and other structures that resulted from the animal’s activities.

In the case of skolithos, it’s widely believed that a vermiform (resembling a worm) animal created the straight, vertical, tube structures. These worm-like critters probably lived by filtering plankton from the turbulent water of a shallow marine environment. The vertical tubes may have been a dwelling place to retreat to, though their specific purpose is not known.

In the above picture, captured in north central Arkansas, a sandstone has weathered to reveal skolithos traces permeating the approximately 460 million year old rock. This example is from an exposure of the St. Peter Formation, Buffalo National River Park, Marion County, Arkansas.

Zinc ore collected in 1943 from the Rush Creek Mining District, Marion County, Arkansas. The brown mineral is sphalerite: an ore of zinc. The pink mineral is dolomite – it’s pretty, but not economically valuable. They were both deposited on the gray dolostone; you can just make it out on the right, in back.

Zinc deposits are found throughout northern Arkansas, commonly with the lead mineral, galena. They’re most abundant in Marion County, in a two mile stretch of rugged terrain, along Rush Creek, where 4 faults come together. That area was mined for lead and zinc in the late 1800s and early 1900s.

It’s typical to find rich ore deposits in rock that’s been fractured by faulting. The fractures facilitate migration of mineral-rich ground water which deposits the ore minerals in the fractures. It’s hard to see in the picture, but the fractured dolostone rock, in this specimen, is bound together by the sphalerite and dolomite minerals.